HttpClient在Web调用中具有广泛的应用，而为它添加默认请求头是我们经常遇到的需求，本文介绍4种为HttpClient添加默认请求头的方式。下面话不多说了，来一起看看详细的介绍吧
第一种方式
直接在创建的HttpClient对象的DefaultRequestHeaders集合中添加报头。
class Program { static Task Main()=> SendAsync1(); private static async Task SendAsync1() { var httpClient = new HttpClient(); AddDefaultHeaders(httpClient); await httpClient.GetStringAsync("http://localhost:5000/"); } private static void AddDefaultHeaders(HttpClient httpClient) { httpClient.DefaultRequestHeaders.Add("x-www-foo", "123"); httpClient.DefaultRequestHeaders.Add("x-www-bar", "456"); httpClient.DefaultRequestHeaders.Add("x-www-baz", "789"); } }
第二种方式
对于.NET Core应用来说，我们更推荐的做法是采用依赖注入的方式，利用IHttpClientFactory来创建HttpClient对象，那么我们在进行相关服务注册的时候就可以设置默认请求报头。
class Program { static Task Main()=> SendAsync2(); private static async Task SendAsync2() { var services = new ServiceCollection(); services.AddHttpClient("", AddDefaultHeaders); var httpClient = services .BuildServiceProvider() .GetRequiredService<IHttpClientFactory>() .CreateClient(); await httpClient.GetStringAsync("http://localhost:5000/"); } private static void AddDefaultHeaders(HttpClient httpClient) { httpClient.DefaultRequestHeaders.Add("x-www-foo", "123"); httpClient.DefaultRequestHeaders.Add("x-www-bar", "456"); httpClient.DefaultRequestHeaders.Add("x-www-baz", "789"); } }
第三种方式
由于HttpClient在发送请求的时候会利用DiagnosticSource对象发送相应的诊断事件，并且将作为请求的HttpRequestMessage对象作为请求事件内容负载。我们可以订阅该事件，在请求被发送之前将其拦截下来，并添加相应的请求头即可。
class Program { static Task Main()=> SendAsync3(); private static async Task SendAsync3() { Func<object, HttpRequestMessage> requestAccessor = null; DiagnosticListener.AllListeners.Subscribe(listener => { if (listener.Name == "HttpHandlerDiagnosticListener") { listener.Subscribe(kv => { if (kv.Key == "System.Net.Http.HttpRequestOut.Start") { requestAccessor ??= BuildRequestAccessor(kv.Value.GetType()); var request = requestAccessor(kv.Value); AddDefaultHeaders(request); } }); } }); var httpClient = new HttpClient(); await httpClient.GetStringAsync("http://localhost:5000/"); static Func<object, HttpRequestMessage> BuildRequestAccessor(Type payloadType) { var property = payloadType.GetProperty("Request", BindingFlags.Instance | BindingFlags.Public); var payload = Expression.Parameter(typeof(object)); var convertedPayload = Expression.Convert(payload, payloadType); var getRequest = Expression.Call(convertedPayload, property.GetMethod); var convert = Expression.Convert(getRequest, typeof(HttpRequestMessage)); return Expression.Lambda<Func<object, HttpRequestMessage>>(convert, payload).Compile(); } } private static void AddDefaultHeaders(HttpRequestMessage request) { request.Headers.Add("x-www-foo", "123"); request.Headers.Add("x-www-bar", "456"); request.Headers.Add("x-www-baz", "789"); } }
第四种方式
上面这种方式可以采用强类型编程方式，具体的代码如下。
class Program { static Task Main()=> SendAsync4(); private static async Task SendAsync4() { DiagnosticListener.AllListeners.Subscribe(listener => { if (listener.Name == "HttpHandlerDiagnosticListener") { listener.SubscribeWithAdapter(new HttpClientListener()); } }); var httpClient = new HttpClient(); await httpClient.GetStringAsync("http://localhost:5000/"); } private sealed class HttpClientListener { [DiagnosticName("System.Net.Http.HttpRequestOut.Start")] public void OnSend(HttpRequestMessage request) => AddDefaultHeaders(request); //Must subscribute the System.Net.Http.HttpRequestOut event. [DiagnosticName("System.Net.Http.HttpRequestOut")] public void OnSend() { } } private static void AddDefaultHeaders(HttpRequestMessage request) { request.Headers.Add("x-www-foo", "123"); request.Headers.Add("x-www-bar", "456"); request.Headers.Add("x-www-baz", "789"); } }